This application claims priority from Japanese Patent Application Nos. P2000-269298 filed Sep. 5, 2000, P2000-269274 filed Sep. 5, 2000, and P2000-269261 filed Sep. 5, 2000; the disclosures of which are herein expressly incorporated by reference to the extent permissible by law.
The present invention relates to a semiconductor thin film applicable to thin film transistors (TFTs) used for liquid crystal displays, memories, and other electronic devices and a fabrication method thereof; an apparatus for fabricating a single crystal semiconductor thin film; and a method of fabricating a single crystal thin film, a single crystal thin film substrate, and a semiconductor device using the substrate.
As semiconductor thin films such as silicon thin films formed on insulating substrates, there have been known an SOI (Silicon On Insulator) structure, and amorphous silicon thin films or polycrystalline silicon thin films formed on glass substrates which have been practically used for liquid crystal displays.
The SOI structure is often formed by way of various steps including a step of sticking single crystal silicon wafers to each other and a step of polishing them, and since the SOI structure basically uses a single crystal silicon wafer, a substantially perfect single crystal portion of the SOI structure can be typically used for a channel portion of an active device of a thin film transistor (TFT). Accordingly, the device thus fabricated can exhibit good electronic characteristics, for example, a high mobility. The method of fabricating the SOI structure, however, requires various steps, for example, a step of sticking single crystal silicon wafers to each other and a step of polishing them, thereby causing disadvantages that the number of steps is increased to prolong the production time, and also the production cost is raised.
On the contrary, there has been known a method of forming a crystallized silicon thin film by depositing a source gas obtained by mixing hydrogen and SiF4 to silane gas on a substrate in accordance with a low-pressure CVD process or a plasma CVD process, and a method of forming a crystallized silicon thin film by forming an amorphous silicon thin film as a precursor on a substrate and crystallizing the amorphous silicon thin film. In the former deposition method in which crystallization of silion proceeds along with deposition of the silicon thin film, since the substrate temperature is required to be kept at a relatively high temperature, more specifically, 600xc2x0 C. or more, the substrate must be made from an expensive material withstanding a high temperature such as quartz. In this method, the use of an inexpensive glass substrate may give rise to a problem that the substrate may be deformed or distorted because of its poor heat resistance. With respect to the latter method, as a process of crystallizing an amorphous silicon thin film formed on a substrate, there has been known a solid-phase growth process of annealing the substrate, on which the amorphous silicon thin film has been formed, for a long time (for example, 20 hr). Such an annealing process, however, has a problem that since it takes a long time, the practical utility is poor and also the production cost is raised. To solve these problems, there has been actively studied and developed a method of crystallizing a non-single crystal thin film by irradiation of laser beams emitted from an excimer laser.
This laser irradiation method involves forming an amorphous silicon thin film or a polycrystalline silicon thin film on a substrate, and heating the thin film by irradiation of laser beams emitted from an excimer laser, thereby crystallizing the thin film. For example, in the case of using an XeCl excimer laser, since an emission wavelength is 308 nm and an absorption coefficient is about 106 cmxe2x88x921, the laser energy is absorbed in a region having a depth of about 10 nm from the surface of an amorphous silicon thin film, with a result that the substrate temperature is little raised, and only a portion near the surface of the amorphous silicon thin film is crystallized.
The technique of melting a non-single crystal thin film by irradiation of laser beams emitted from an excimer laser and recrystallizing the melted thin film can grow polycrystalline silicon grains in an amorphous silicon thin film or a polycrystalline silicon thin film; however, it is very difficult to stably control a crystal quality of the thus formed thin film on the basis of the number of shots of laser beams, thereby tending to cause a variation in threshold voltage of a thin film transistor as a final product.
By the way, in the case of using a PECVD (Plasma-Enhanced CVD) system for forming an amorphous semiconductor thin film on a substrate, the film contains hydrogen in an amount of about 2 to 20 atomic %. In this case, the substrate on which the thin film has been formed is put in an electric furnace and is subjected to a degassing treatment, for example, at 420xc2x0 C. for about 2 hr. The hydrogen concentration in the film is reduced to less than 2 atomic % by the degassing treatment.
Such a degassing (annealing) treatment in an electric furnace for removing hydrogen contained in the film has a problem that since the substrate must be annealed, for example, at 420xc2x0 C. for about 2 hr, the productivity is degraded, and further, the substrate may be deformed due to heat caused by the degassing treatment and a contaminant from glass may be diffused in the thin film.
An object of the present invention is to provide a semiconductor thin film which has a performance very higher than that of a related art polycrystalline thin film and is suitable for fabricating a device having stable characteristics and which is fabricated for a sufficiently short time, and a method of and an apparatus for fabricating the semiconductor thin film.
Another object of the present invention is to provide a semiconductor device and a single crystal thin film substrate fabricated by using such a semiconductor thin film having a high crystal quality.
A further object of the present invention is to provide a method of fabricating a semiconductor thin film, which is capable of preventing the explosion of a thin film during fabrication steps and removing hydrogen from the thin film for a short time, and an apparatus for fabricating a single crystal semiconductor thin film.
To solve the above-described problems, the present inventors have found that one of the causes of obstructing the enlargement of sizes of crystal grains in a polycrystalline thin film is dependent on the manner of irradiating the thin film with laser beams, and eventually created an innovative semiconductor thin film quite different from the related art polycrystalline thin film and a fabrication thereof. More specifically, the present inventors have found that a crystallized semiconductor thin film can be formed by crystallizing a non-single crystal thin film by laser irradiation under irradiation conditions such that polycrystalline grains aligned in an approximately regular pattern are formed on the thin film, and heat-treating the thin film with the surface state having projections kept as it is, thereby promoting crystallization of the thin film.
Accordingly, to solve the above-described technical problems, according to a first aspect of the present invention, there is provided a method of fabricating a single crystal thin film, including the steps of: forming a non-single crystal thin film on an insulating base; subjecting the non-single crystal thin film to a first heat-treatment, thereby forming a polycrystalline thin film in which polycrystalline grains are aligned in an approximately regular pattern; and subjecting the polycrystalline thin film to a second heat-treatment, thereby forming a single crystal thin film in which the polycrystalline grains are bonded to each other.
To largely grow a single crystal region in which polycrystalline grains have been bonded to each other, it may be preferred that adjacent polycrystalline grains be in a state being easy to be bonded to each other. From this viewpoint, according to the present invention, since polycrystalline grains are aligned in an approximately regular pattern, it is possible to obtain common crystal directivities of the polycrystalline grains, for example, common crystal orientation planes such as the (100) plane at the time of recrystallization after heat-treatment, and hence to attain smooth bonding of the polycrystalline grains by making use of the order thereof. Accordingly, at the time of heat-treatment, the bonding of the polycrystalline grains will be easily promoted, to convert the polycrystalline film into a single crystal film.
According to a second aspect of the present invention, there is provided a method of fabricating a single crystal thin film, including the steps of: forming a non-single crystal thin film on an insulating base; and irradiating the non-single crystal thin film with laser beams, thereby converting the non-single crystal thin film into a single crystal thin film.
With this configuration, crystals in the non-single crystal thin film are grown into a single crystal film by irradiating the non-single crystal thin film with laser beams, whereby the non-single crystal thin film is converted into the single crystal thin film.
According to a third aspect of the present invention, there is provided a method of fabricating a single crystal thin film, including the steps of: forming a non-single crystal thin film on an insulating base; subjecting the non-single crystal thin film to a first heat-treatment, to introduce a common boundary condition, thereby forming a polycrystalline thin film; and subjecting the polycrystalline thin film to a second heat-treatment, thereby forming a single crystal thin film in which polycrystalline grains are crystallized.
With this configuration, since the common boundary condition is introduced to the polycrystalline grains by the first heat-treatment, it is possible to obtain common crystal directivities of the polycrystalline grains, for example, common crystal orientation planes such as the (100) plane at the time of recrystallization after the first heat-treatment, and hence to attain smooth bonding of the polycrystalline grains by making use of the order thereof. Accordingly, at the time of the second heat-treatment, the bonding of the polycrystalline grains can be easily promoted, to convert the polycrystalline film into a single crystal film.
According to a fourth aspect of the present invention, there is provided a single crystal thin film substrate including: an insulating base; and a single crystal thin film formed on the insulating base by single-crystallization using laser irradiation. With this configuration, the crystallization for forming the single crystal thin film substrate is performed by aligning polycrystalline grains in a polycrystalline thin film in an approximately regular pattern, and heat-treating the polycrystalline thin film.
According to a fifth aspect of the present invention, there is provided a semiconductor device formed by using the above single crystal thin film substrate.
According to a sixth aspect of the present invention, there is provided a semiconductor thin film formed on an insulating base, including: micro-projections formed on the surface of the semiconductor thin film.
The micro-projections on the semiconductor thin film according to the present invention are portions where boundaries of polycrystalline grains in the polycrystalline thin film, formed during fabrication steps, collide with and overlap each other. Such micro-projections can be observed by a microscopic photograph as will be described later. The height of each micro-projection may be in a range of 20 nm or less, preferably, 10 nm or less, more preferably, 5 nm or less; the diameter of each micro-projection may be in a range of 0.1 xcexcm or less, preferably, 0.05 xcexcm or less; and a radius of curvature of each micro-projection may be in a range of 60 nm or more, preferably, 180 nm or more, more preferably, 250 nm or more. The density of micro-projections is in a range of 1xc3x971010 pieces/cm2 or less, preferably, 1xc3x97109 pieces/cm2 or less, more preferably, 5xc3x97108 pieces/cm2 or less. The size of a single crystal region may be in a range of 1xc3x9710xe2x88x928 cmxe2x88x922 or more, preferably, 1xc3x9710xe2x88x927 cmxe2x88x922 or more. The single crystal region is not required to be formed on the entire surface of an insulating base but may be present on part of a polycrystalline thin film.
According to a seventh aspect of the present invention, there is provided a method of fabricating a semiconductor thin film, including the steps of: forming a non-single crystal thin film on an insulating base; subjecting the non-single crystal thin film to a first heat-treatment, thereby forming a polycrystalline thin film; and subjecting the polycrystalline thin film to a second heat-treatment, thereby forming a crystallized semiconductor thin film; wherein projections on the surface of the crystallized semiconductor thin film are lower than projections on the surface of the polycrystalline thin film.
With this configuration, at the first heat-treatment, the projections are formed on the surface of the polycrystalline thin film. The projections uplifted from the surface are formed by overlapping boundaries of the polycrystalline grains. The heat-treatment for forming the projections on the surface of the polycrystalline is exemplified by irradiation of laser beams emitted from an excimer laser. At the second heat-treatment, the projections on the surface of the single crystal thin film are lower than the projections on the surface of the polycrystalline thin film, so that at least part of the portions where the boundaries are overlapped to each other substantially disappear. This makes it possible to obtain a thin film including a single crystal region having an excellent crystal quality.
According to an eighth aspect of the present invention, there is provided a method of fabricating a semiconductor thin film, including the steps of: forming a non-single crystal thin film on an insulating base; subjecting the non-single crystal thin film to a first heat-treatment, thereby forming a polycrystalline thin film; and subjecting the polycrystalline thin film to a second heat-treatment, thereby forming a crystallized semiconductor thin film; wherein a radius of curvature of each of projections on the surface of the crystallized semiconductor thin film is larger than a radius of curvature of each of projections on the surface of the polycrystalline thin film.
With this configuration, at the second heat-treatment, the radius of curvature of each of the projections on the surface of the crystallized semiconductor thin film is larger than the radius of curvature of each of the projections on the surface of the polycrystalline thin film, so that at least part of the portions where the boundaries are overlapped to each other substantially disappear. This makes it possible to obtain a semiconductor thin film having an excellent crystal quality.
According to a ninth aspect of the present invention, there is provided a method of fabricating a semiconductor thin film, including the steps of: forming a non-single crystal thin film on an insulating base; subjecting the non-single crystal thin film to a first heat-treatment, thereby forming a polycrystalline thin film in which polycrystalline grains are aligned in an approximately regular pattern; and subjecting the polycrystalline thin film to a second heat-treatment, thereby forming a semiconductor thin film in which micro-projections are each formed at a boundary position among at least three or more of the polycrystalline grains; wherein a height of each of the micro-projections is in a range of 25 nm or less or a radius of curvature of each of the micro-projections is in a range of 60 nm or more.
With this configuration, the polycrystalline thin film, in which the polycrystalline grains have been aligned in an approximately regular pattern by the first heat-treatment, is then subjected to the second heat-treatment. Accordingly, at least part of the portions where the boundaries are overlapped to each other substantially disappear. This makes it possible to obtain a crystallized semiconductor thin film having an excellent crystal quality.
According to a tenth aspect of the present invention, there is provided each of a semiconductor device and a substrate using the above single crystal thin film having micro-projections as part of the structure.
According to an eleventh aspect of the present invention, there is provided a method of fabricating a semiconductor thin film on a base, including the steps of: forming a hydrogen containing non-single crystal semiconductor thin film; subjecting the hydrogen containing non-single crystal thin film to a first heat-treatment, thereby removing hydrogen therefrom; continuously subjecting the non-single crystal thin film from which hydrogen has been removed to a second heat-treatment, thereby forming a polycrystalline film in which polycrystalline grains are aligned in an approximately regular pattern.
With this configuration, since the polycrystalline film in which polycrystalline grains are almost aligned is formed by the second heat-treatment, the crystallized semiconductor thin film in which crystallization has proceeded can be formed. After hydrogen is removed by the first heat-treatment, the polycrystalline film is formed by the second heat-treatment which is continuous to the first heat-treatment. Accordingly, it is possible to eliminate the need of opening the substrate to atmosphere and hence to shorten the time required for the first and second heat-treatments. Further, since removal of hydrogen is performed prior to heat-treatment, it is possible to prevent the explosion of the film.
According to a twelfth aspect of the present invention, there is provided a method of fabricating a semiconductor thin film on a base, including the steps of: forming a hydrogen containing non-single crystal semiconductor thin film; subjecting the hydrogen containing non-single crystal thin film to a first heat-treatment, thereby removing hydrogen therefrom; continuously subjecting the non-single crystal thin film from which hydrogen has been removed to a second heat-treatment, thereby melting and recrystallizing the non-single crystal thin film; and subjecting a polycrystalline film formed by melting and recrystallization to a third heat-treatment, thereby converting the polycrystalline film into a single crystal film.
With this configuration, after the non-single crystal thin film is melted and recrystallized, the third heat-treatment is performed to convert the polycrystalline film formed by melting and recrystallization into a single crystal film. After hydrogen is removed, the second heat-treatment is continuously performed. Accordingly, it is possible to eliminate the need of opening the substrate to atmosphere and hence to shorten the time required for the first and second heat-treatments. Further, since removal of hydrogen is performed prior to heat-treatment, it is possible to prevent the explosion of the film.
According to a thirteenth aspect of the present invention, there is provided an apparatus for fabricating a single crystal semiconductor thin film on a base, including: thin film forming means for forming a hydrogen containing non-single crystal thin film on the base; first heat-treatment means for subjecting the hydrogen containing non-single crystal thin film to a first heat-treatment, thereby removing hydrogen therefrom; and second heat-treatment means for continuously subjecting the non-single crystal thin film from which hydrogen has been removed to a second heat-treatment, thereby melting and recrystallizing the non-single crystal thin film.
According to a fourteenth aspect of the present invention, there is provided an apparatus for fabricating a single crystal semiconductor thin film on a base, including: thin film forming means for forming a hydrogen containing non-single crystal thin film on the base; first heat-treatment means for subjecting the hydrogen containing non-single crystal thin film to a first heat-treatment, thereby removing hydrogen therefrom; and second heat-treatment means for continuously subjecting the non-single crystal thin film from which hydrogen has been removed to a second heat-treatment, thereby forming a polycrystalline film; and third heat-treatment means for subjecting the polycrystalline film to a third heat-treatment, thereby converting the polycrystalline film into a single crystal film.
With these configurations, after hydrogen is removed, the second heat-treatment is continuously performed. Accordingly, it is possible to eliminate the need of opening the substrate to atmosphere and hence to shorten the time required for the first and second heat-treatments.